Copper and copper alloys are widely used in electrical and electronic applications due to excellent electrical characteristics. The mechanical properties are less than ideal. Copper is relatively soft. Connectors and other electronic components are subject to frictional wear. The metal readily oxidizes and the oxides are poor conductors of electricity. The formation of oxides on the surface of a connector leads to an increase in the contact resistance which is not desirable.
Oxidation of copper can be minimized by coating with a second metal such as tin or gold. The coatings have a minimal effect on the hardness or wear resistance of the substrate.
The addition of alloying additives to copper can increase wear resistance and hardness as well as improve oxidation resistance. However, the additives usually cause a decrease in electrical conductivity.
It is therefore desirable to develop a means to increase the wear resistance, hardness and oxidation resistance of copper or a copper alloy without significantly decreasing the electrical conductivity. This goal is accomplished by surface modification.
Surface modification affects only the outer surface of the substrate. The affected depth is from a few angstroms to less than about 1 micron. Among the methods of surface modification are carburization and nitriding. Steels and titanium alloys are hardened by these processes. Surface modification generally involves heating the metal substrate to a temperature below the melting point in a stream of reaction gas. Alternatively, ions of the reaction gas may be implanted in the surface by ion implantation.
U.S. Pat. No. 4,693,760 issued to Sioshansi discloses a process and fixture for modifying the surface of a titanium alloy with carbon or nitrogen by ion implantation. An article by Oliver et al entitled The Wear Behavior of Nitrogen-Implanted Metals details how the wear characteristics of titanium alloys, a hard chromium plate and ferrous metals are improved by nitriding the surface by ion implantation.
Nitriding of copper and copper alloys to improve surface properties is difficult. The attraction between copper and nitrogen is minimal. Nitriding by conventional means does not produce significantly improved surfaces.
A boride dispersed copper alloy having improved surface properties has been produced. The process is disclosed in U.S. Pat. No. 4,436,560 to Fujita et al. A copper substrate is alloyed with a metal which reacts with boron. Alternatively, the surface of the alloy is coated with a reactive metal. Boron is then brought into contact with the surface by immersion in molten boron salt, fluid bed deposition or vapor deposition. The resultant surface has a dispersion of fine boride particles in a copper matrix. The dispersoids occupy from 1 to 50% of the surface volume.
This technique is only suitable when the precipitate former (boron) is soluble in the host matrix (copper). Nitrogen is not significantly soluble in copper. The process is not suitable for forming nitride precipitates at the surface of copper or a copper alloy.